»
White Dwarfs

It is evident from the photometric observations of both ROSAT and EUVE surveys of white dwarfs, that many very hot white dwarfs must contain significant quantities of heavy elements, in addition to He, in their atmospheres. The nature of at least some of the sources of opacity has been revealed by UV observations but EUV spectroscopy has been expected to add important new information on elemental abundances and, possibly, their distribution within the stellar photosphere. Indeed, the...

Hence, for an event to last for a long time a mechanism is required that will compensate for the radiative losses. Small-scale flaring processes cannot support additional heating with a total energy 1035 ergs. Therefore, Katsova et al. (1996) consider processes occurring on a larger scale, comparable to or greater than the stellar radius. The first pulse of the EUV light curve is considered to be caused by an impulsive flare. The gradual decay after this can be...

In 1975, an opportunity arose to perform an extended search for EUV sources of cosmic radiation on the Soviet-American Apollo-Soyuz mission (officially termed the Apollo-Soyuz Test Project, or ASTP). In July of that year, US and Soviet astronauts were to link up in space in a demonstration of international good will. NASA realised that, in addition to the political benefits of the flight, some science instruments could 'piggyback' on the Apollo capsule, and solicited proposals for experiments...

The main components of EUVE have been described in detail in chapter 4 with the exception of the spectrometer. This instrument made use of part of the converging beam of the Wolter type II deep survey telescope, intercepting this with three variable line space reflection gratings (Hettrick et al. 1985). Such gratings are mechanically ruled plane gratings with a line density that varies continuously from one end of the grating to another to correct first order aberration arising from the varying...

The overwhelming majority of known white dwarfs are isolated stars. This is perhaps not surprising, as they have mostly been identified through optical blue star colour and proper motion surveys (e.g. Green et al. 1986b). Thus there exists an inherent bias against the detection of white dwarfs in unresolved binary systems, particularly if there is no active mass transfer which might otherwise draw the observer's attention, as in the cataclysmic variables. For example, the spectrum of any...

So far, our discussion has concentrated on the development of EUV astronomy in the context of the detection of EUV sources. However, the EUV sky background plays a significant role in the process of source detection since the source flux must be sufficient to render the source visible above the background. There are two separate sources of background the yet to be detected cosmic sky background, arising from hot gas in the ISM, and emission arising from the Earth's local environment. The cosmic...

Late-type stars constitute the largest single class of objects in the WFC EUV source catalogue. With EUV luminosities in the range 1027-1028 ergs s-1 (the EUV luminosity of the Sun is 1027 ergs s-1), one to two orders of magnitude below those of typical white dwarfs, this source population is expected to occupy a smaller volume within 10 to 25 pc of the Sun. Hence, most of these objects should be found in the third catalogue of nearby stars (CNS3 Gleise and Jahreiss 1991), the most complete...

The WFC was designed to cover the EUV range during the ROSAT mission, principally from 60 to 200 A during the survey itself, but with a capability of observing at longer wavelengths, to 600 A, during the pointed phase of the mission. The instrument consisted of a grazing incidence telescope comprising a set of three Wolter-Schwarzchild Type I gold-coated aluminium mirrors (Willingale 1988), with a MCP detector in the focal plane (Barstow et al. 1985 Fraser et al. 1988 Barstow and Sansom 1990)....

Most of the work dealing with the EUV spectroscopy of stellar coronae has been phenomeno-logical, associated with determining plasma temperatures, emisson measures, densities and abundances. Important discussions concerning departures from photospheric abundance in the coronae, dealing with the FIP effect or the MAD syndrome, have begun to establish patterns for stars with different levels of activity. However, little work has been done to link the observational parameters to physical...